Bias

Bias is a systematic error that leads to an incorrect estimate of effect or association. Many factors can bias the results of a study such that they cancel out, reduce or amplify a real effect you are trying to describe.

Epidemiology categorises types of bias, examples are:

Selection bias - e.g. study of car ownership in central London is not representative of the UK.

Observation bias (recall and information) - e.g. on questioning, healthy people are more likely to under report their alcohol intake than people with a disease.

Observation bias (interviewer) - e.g. different interviewer styles might provoke different responses to the same question.

Observation bias (misclassification) - tends to dilute an effect

Losses to follow up - e.g. ill people may not feel able to continue with a study whereas health people tend to complete it.